Problem: Before the previous stop there were 84 people riding on a train. 39 people ended up getting off at the stop. How many people are riding the train now?
Explanation: The number riding the train now is the difference between the number who were riding and the number who got off at the previous stop. The difference is $84 - 39$ people on the train. $84 - 39 = 45$ people are on the train.